Method and apparatus of measuring the activity of radioactive materials



Aplfn 45, E95@ G. H. WANNIER 2,5@29633 METHOD AND PPRTUS OF' MEASURINGTHE ACTIVTY OF RADIOACTIVE MATERIALS Filed Sept. 22, 1948 Ff f Y Ff@INVENTOR. @H'y H WAN/VIER Patented Apr. 4, 1950 METHOD ANDl APPARATUS OFMEASURING` THE ACTIVITY F RADIOACTIVE MATE- RIALSv Gregory H. Wannir,Pitman, N. J., assigner to.

Socony-VacuumAOil Company, Incorporated, a corporation of New 'YorkApplicatonSeptember 22, 1948, SerialNo. 50,594

3 Claims.

This invention relates generally to the analysis of radioactivesubstances and more particularly to apparatus for. measuring theactivity of a sample of the substance.

When ther activity ofk a radioactive sample is determined by employing acounter tube, the number of counts which are observed per second dependsupon several outside factors. An accurate control of these factors isessential if the activity determination is to be accurate. The mostimportant. of these factors` is the counting geometry. y

This-inventionlsolves the problem of providing a Well-defined countinggeometry. When aradioactive point source is placed in front of acounter, Figure. 1 is a schematic illustration of the arthe counterresponse is proportional to the solid rangement of a sample and counterthat is in angle subtended by the counter window at the accordancewiththe prior art; source. This geometrical factor can becalcu- Figure 2is a schematic illustration ofv a sample lated for point sources,'butdifficulties are enand counter arranged in accordance with the counteredif the sourceis extended over a subpresent invention; and stantial area.Figure 3 is a groupfof curves which illustrate According to the priorart it is common practhe manner in which the solid angle subtended ticeto deposit radioactive. material y on an inby the counter window variesfrom point to point active backing which is usually a plane in the overthe surfacelof a sample having the convenform of a' disk. If such aplane disk is placed '2.5 tional arrangement. in front of a counter, thesolid angle subtended Referring to the'drawings in detail, particularlyat the source by the counter window varies from Figure l, there is showna backing element l0 point to point over the surface ofthe sample. whichis formed of an inactive metal. Element It is possible, in principle, tocalculatethe'mean I0, an edge of which is shown, is a plane vand solidangle, but such calculation presumes that may take the form of a disk.Element ID has the radioactive material is distributed uniformlydeposited on one face thereof a sample I-I of over the surface. Such acondition is difficult to radioactive material, the activity of which itis realize in practice. It follows that this geometridesired to.measure. Adjacent` the sample II cal factor can vary from sample tosample, even there is disposed a counter tube I2, a fragment though thesamples are of the same size and of which is shown in section. Thefragment placed in the same position on the disk. In shown may have aglass wall with a window I3 practical cases this geometrical error isthe main formed therein. Window I3 may be sealed by error of the entiremeasurement. any material that is transparent to the radiation Thisinvention eliminates this geometrical error emitted by the sample I I.Suitable window maby making the surface which supports the radioterialsare glass, mica, etc. Window I3 is usually active material curved,rather than a plane. The circular in form. curvature is made such thatthe solid angle sub- A comparison of the angles A and B will show tendedby the counter window at the source is the manner in which the solidangle at the surthe same for every point on the surface of the face ofthe sample subtended by the window I3 sample of radioactive material.The mean solid varies from point to point on the surface of the angle isthen automatically also the same, and Sample II. It is obvious theWindow I3 will subthe result so obtained is independent of Whether tendthe greatest angle, angle A, at the center the radioactive material isdistributed evenly of the surface of the sample II and that the over thesurface or not. angle subtended by the window I3 at the edge Thereforethe primary object of this invention of the sample, angle B, will be thesmallest. The is to provide a method and apparatus for elimimannerinwhich the solid angle varies from point nating from an activitymeasurement of a radioto point over the surface of the sample II mayactive sample of material, the geometrical error. be read off the curvesof Figure 3. Therefore More specifically the present invention has for uit is clear that a significant geometrical error will an object to soarrange the sample undergoing test with respect to the window of thecounter kthat the solid angle at the source subtended by the window isthe same for every point on the surface of the source. Another specicobject of this invention is the. provision of a curved surface, on whichthel sample can be deposited, whose curvature. and Whose distancefromthe Window of the counter are so selected that the solid angle, onthe surface of the sample, subtended by the windowis thesame for allpoints on the surface of the sample. Other objects and advantages of thepresent invention will. become apparent from the-following detaileddescription when consideredwith the drawings, in which be introduced bythe use of this arrangement.

posited over its surface the sample Il whose activity it is desired tomeasure.

the counter tube I2 and window [e3 with respect The location of f l0 fto the center of the surfaceof the sample Il is the same as in Figure l.However, in the arrangement of Figure 2, the angles subtended at allpoints on the surface off .the sample are equal. This can be seen whenangles A and B are com-I pared. The distance between the center of thesample and the center of-fthe 4counter window and the curvature of thesample are so selected sample holder disposed adjacent said window, saidthat the angles from any points on the sample that are subtended by thewindow are always the same size. e

An example will 4serve to illustrate the difference between the oldarrangement and the new. According to the prior art arrangement, if acounter having a circular window of 1.5 cm. diameter is used and a planecircular source of 1 cm. diameter is placed directly opposite the windowat a distance of 0.75 crn., then the solid angle subtended by the windowvaries by approximately 25% from the center to the edge 0f the source,as may be seen from the curves or Figure 3. If, on the contrary, thesource be given the shape of a spherical cap, as shown in Figure 2,whose radius of curvature is l cm., then this variation is reduced to anegligible amount. This specic example is only a special case of thegeneral prin` ciple stated above. Other examples can be made up readilyby consulting the curves of Figure 3.

In practicing the present invention care must be exercised to preventradiation from entering the counter tube except through the Window i3.

To this end a lead shield l5 is placed between all parts of the tube andthe sample exceptl between the window and the sample.

It is obvious that the element IU need not be formed of metal, forexample, blotting paper, pressed nbre, or other materials may be used solong as they are not radioactive.

I claim:

1. An apparatus for measuring the activity of a sample of radioactivematerial that comprises a counter tube, means deiining a window in thecounter tube, means transparent to radiation emitted by the sample forsealing said window, a sample holder disposed adjacent said window, saidsample holder having a concave recess in the side thereof adjacent toand in axial alignment with said window, a coating of the radioactivematerial on the surface dening the concave recess, the curvature of therecess bearing a relation to the axial distance between the sample andthe window such that the solid angle subtended by the window at anypoint on the concave surface fof the sample is a constant.

2. An apparatus for measuring the activity of a sample of radioactivematerial that comprises a "1 counter tube, means dening a window in the'counter tube, means transparent to radiation emitted by the sample forsealing said window,

imeans disposed about the window for shielding the remainder of the tubefrom radiation, a

sample holder having a concave recess in the side thereof adjacent toand in axial alignment with said window, a coating of the radioactivematerial on the surface defining the concave recess, the curvature ofthe recess bearing a relation to the axial distance between the sampleand the window such that the solid angle subtended by the window at anypoint on the concave surface of the sample is a constant.

3. An apparatus for measuring the activity of a sample of radioactivematerial that comprises a counter tube, means defining a window in thecounter tube, means transparent to radiation emitted by the sample forsealing said window, a sample holder disposed adjacent said window, saidsample holder having a concave recess in the side of the holder adjacentto and in axial alignment with said window, a coating of the radioactivematerial on the surface defining the concave recess, the curvature ofthe recess bearing a relation to the axial distance between the sampleand the window and to the area of the window such that the solid anglesubtended by the window-at any point on the concave surface of thesample is a constant.

GREGORY H. WANNIER.

REFERENCES CITED v The following references are of record in the file ofthis patent:

UNITED STATES PATENTSl Number

